CD8-dependent CTL require co-engagement of CD8 and the TCR for phosphatidylinositol hydrolysis, but CD8-independent CTL do not and can kill in the absence of phosphatidylinositol hydrolysis.

Most instances of MHC class I recognition and target cell killing by CD8+ CTL require the involvement of CD8. The role of CD8 in these events may be both for adhesion of the CTL with the APC, as well as for signal transduction through the TCR. The precise mechanism by which CD8 mediates signal transduction remains enigmatic. Similarly, it is unclear whether only the CD8 molecules which bind to the same class I molecule as the TCR contribute to signaling in the T cell responding to antigen. We have investigated the requirement for co-engagement of CD8 and the TCR in the induction of the hydrolysis of phosphatidylinositol-4,5-bisphosphate (PIP2) during the interaction of CTL and APC transfected with either wild-type or mutant (CD8 non-binding) class I molecules. Our results show that for conventional CD8-dependent killing co-engagement of both CD8 and the TCR is required to initiate PIP2 hydrolysis. This requirement for co-engagement, however, can be overcome by a high density of ligand, such as that provided by high concentrations of exogenous peptide. In such situations, the binding of CD8 to non-antigenic class I molecules can elicit PIP2 hydrolysis. Therefore, during interactions between CTL and APC, which generally occur at low concentrations of antigenic peptide, triggering of PIP2 hydrolysis requires TCR and CD8 co-engagement, and the binding of CD8 to non-antigenic class I molecules does not contribute significantly to signaling within the T cell.(ABSTRACT TRUNCATED AT 250 WORDS)